(1) Field of the Invention
The present invention relates to a proportional valve, and more particularly, to a solenoid-operated proportional valve whose lift amount varies in proportion to a value of current supplied to the solenoid.
(2) Description of the Related Art
As a refrigerating cycle of an automotive air conditioning system, for example, there has been known a configuration wherein high-temperature, high-pressure gaseous refrigerant compressed by a compressor is condensed or cooled by a condenser or a gas cooler, the condensed or cooled refrigerant is turned into low-temperature, low-pressure refrigerant by a pressure reducing device, the low-temperature refrigerant is evaporated by an evaporator, the evaporated refrigerant is separated into gas and liquid by an accumulator, and the separated gaseous refrigerant is returned to the compressor. In such systems, a solenoid-controlled proportional valve capable of controlling a valve lift thereof in proportion to a value of current supplied thereto from outside is used as the pressure reducing device.
Conventional proportional valves are generally constructed such that a valve element which lifts in proportion to a value of current supplied to a solenoid moves together with a piston which has a cross-sectional area equal to the valve hole of a valve seat and which is applied with a pressure on an upstream side of the valve element at an end face thereof opposite the valve element. Accordingly, pressures acting upon the valve element and the piston are of the same magnitude but opposite in direction and thus are canceled out. The valve element can therefore be moved only by an urging force exerted by the solenoid, regardless of the magnitude of the pressure on the upstream side of the valve element.
The piston is so constructed as to move together with the valve element while receiving the pressure on the upstream side of the valve element at one end face thereof and the pressure on the downstream side of the valve element at the other end face thereof. Accordingly, a sliding portion of the piston between its both end faces needs to be hydraulically sealed. For such sealing, a labyrinth seal is used (see Japanese Unexamined Patent Publication No. 2002-130870 (FIG. 1), for example). The labyrinth seal is low in sliding resistance and thus is often used as a method for sealing a piston of which the back pressure is to be canceled.
In conventional proportional valves, however, it is generally known that the pressure receiving area of the valve section varies depending upon the lift amount of the valve element, while the pressure receiving area of the piston is fixed. Thus, the back pressure cancellation fails to function properly, giving rise to a problem that the valve element moves by itself to open or close due to the difference between the pressure receiving areas.
Also, the piston has a labyrinth seal for sealing between the space on the upstream side of the valve section and the space on the downstream side of same. Since the labyrinth seal is unable to provide perfect seal, however, the refrigerant leaks from the labyrinth seal when the valve section is fully closed, for example, giving rise to a problem that the controllability lowers due to such internal leakage.